This invention relates generally to the fields of optical modulators, light valves, electro-optic filters, projection and flat panel display devices. More specifically, this invention relates to an optical modulating device, light-valve, display, or optical filter, which uses variation in incident angle to exploit color-selective absorption at a metal-dielectric interface by surface plasmons.
In xe2x80x9cVoltage-Induced Color-Selective Absorption with Surface Plasmons,xe2x80x9d Applied Physics Letters, Volume 67, No. 19, Nov. 6, 1995, pp. 2759-2761, Yu Wang reported on the phenomenon of voltage-induced color-selective absorption at a metal/liquid crystal interface with surface plasmons. The surface plasmon, a collective excitation of electrons, absorbs all incident light at the resonance frequency of the plasmon. When incident p-polarized light is absorbed at the surface plasmon resonance, the reflected light will show a color complementary to that which is absorbed.
Wang teaches that by using a liquid crystal, whose dielectric constant varies with applied voltage, one can change the resonance frequency of the surface plasmon, which in turn provides a concomitant change in absorption. Changing the dielectric constant of the liquid crystal through the application of voltage results in the reflected light showing a color change.
FIG. 1 schematically shows the prior art as taught by Wang. In this figure, the projection display 10 includes a substrate 11, bottom electrode 12, alignment layers 13, spacers 14, top electrode 15, liquid crystal 16 and seal 17. Alignment layers 13, typically formed by deposition of an oxide or polyimide layer and mechanical rubbing of the surface, are required to be employed to impart a preferred direction to the liquid crystal 16. Spacers 14 are required to set the desired spacing between top electrode 15 and bottom electrode 12. Seal 17 is required to contain liquid crystal 16 between top electrode 15 and bottom electrode 12.
In U.S. Pat. No. 5,570,139, Wang describes a similarly designed liquid crystal embodiment and how this embodiment may be employed in projection display applications.
In U.S. Pat. No. 5,986,808, Wang describes a surface plasmon tunable filter using metallic layers bordering a dielectric region with an adjustable air gap as a dielectric. Wang teaches the use of piezoelectric spacers to physically expand or contract the air gap when a voltage is applied.
Similarly, in U.S. Pat. No. 6,031,653, Wang describes thin-film metal interference filters forming a Fabry-Perot cavity in which piezoelectric spacers are used to control an air gap between two metal films.
The complexities of liquid crystal and incorporated piezoelectric structures pose substantial limitations towards the ability to rapidly and easily manufacture optical modulating devices in great numbers and at once.
This invention provides an improved design for a surface plasmon device by relaxing the need for an electro-optic material, electrical bias, and capacitor device structure. The design also avoids the requirements of liquid crystal configurations, such as spacers, alignment layers, and seals.
The invention is an optical modulating device, capable of being used as a light valve, display, or optical filter, for example, uses variation in incident angle to exploit color-selective absorption at a metal-dielectric interface by surface plasmons.
The invention includes a dielectric layer, at least one metallic layer through which electromagnetic radiation may be transmitted or reflected, and incident and exit layers which are both optically transmissive layers. A beam steering mechanism 14 controls the incident angle 15 of the electromagnetic radiation 13. In one embodiment, an external beam steering mechanism is used to set the incident light angle onto the filter. In another embodiment, the filter is formed as an integral part of, for example, a cantilever. The incident angle is then controlled by the angle of the filter cantilever.